The Holyland in Tumult! Emperor of the Sea Blackbeard Cackles!

On their way to "The Palace of the Sea God" Robin's group meets an entranced Zoro. It looks like Zoro wants to fight so Luffy urges the others to keep going without him. They apprehend the kid but the whistle was what was really stealing their memories. Zoro returns, still entranced, but Luffy's nowhere to be seen.